Not only optical fibers each having a two-layer structure including a core and a cladding, but also optical fibers each including a plurality of claddings and exhibiting lower bending loss have been put into practice.
PATENT DOCUMENT 1 describes a single-mode optical fiber including a highly absorbent outer cladding glass made of quartz glass doped with cobalt oxide using a solution of cobalt oxide, a non-absorbent inner cladding glass, and a non-absorbent core glass.